1. Field of the Invention
The present invention relates to telephone networks, and more particularly to a system and method for accessing, monitoring and testing a telephone network.
2. Description of the Related Technology
The telephone industry has changed drastically since the divestiture of the Bell System. Today, seven regional Bell Operating Companies (RBOCs) and independent telephone companies provide local telephone service within 166 Local Access Transport Areas (LATAs). These companies are forced to rely on interexchange carriers such as AT&T, MCI and Sprint for transmission of calls from one LATA to another. The responsibility for quality and performance of the telephone circuit is thus split between local telephone companies and interexchange carriers.
RBOCs are under pressure for financial performance as independent companies. With rates restricted by utility commissions, and facing rising costs and new competition with restrictions on transmission of calls beyond LATA boundaries, telephone companies find themselves faced with rapid introduction of new technology, smarter business customers, and antiquated network maintenance systems.
Demand for information transmission increased dramatically during the 1980s. At the beginning of the decade most data transmission systems interfaced a predominantly analog network through relatively low speed modems. Those desiring "high speed" transmission generally opted for the 56 kbps rate of the digital data system (DDS).
Considerable pressure for increases in such transmission rates came from the desire to take advantage of the increasing capabilities and computation speeds of computers and other business systems. Improvements in transmission technology during the decade fueled the momentum of the increase in transmission rates. The replacement of copper cables with glass fiber expanded the transmission capacity of outside plants many times over. At the same time, improvements in electronics and coding algorithms yielded terminal equipment designed to take advantage of the enormous increase in bandwidth which accompanied the conversion to fiber optics.
In the absence of a standard, virtually all lightwave vendors chose DS3 (44.736 Mbps) as the interface between the lightwave terminal and the network. FIG. 1 illustrates the prior art North American Digital Hierarchy having a DS0 (64 kbps) level 102, a DS1 (1.544 Mbps) level 104, a DS2 (6.312 Mbps) level 106 and a DS3 (44.736 Mbps) level 108. This hierarchy is defined by ANSI T1.102--1987--"Digital Hierarchy, Electrical Interfaces", The American National Standards Institute, Inc., New York, 1987. DS2 is important as a link between DS1 and DS3. Even though there is little growth in DS2 as a transport medium, the DS2 level exists in every muldem (multiplexer/demultiplexer) or other network element which must interface DS1 and DS3 signals. Although DS0 is essentially confined to digital signals, reference to analog voice frequency signals is included in FIG. 1 because of widespread interfacing of such signals to the DS1 level of hierarchy by digital channel banks.
The transition of telecommunications into the 1990s will thus occur with the DS3 rate used almost universally for interfaces within the network. DS1 transmission between customers and operating companies is now commonplace, and an ever increasing number of customers are seeking to interface with service providers and with other end users at even higher rates. The DS2 rate, seemingly a logical intermediate step between DS1 and DS3, has proved to be uneconomical for transport except in certain special cases. Thus, DS3 is proving to be the underlying building block for high bandwidth, light signals.
FIG. 2 is a prior art, simplified model of a lightwave network 120 showing four example network carriers (Carrier A, Carrier B, Carrier C and Carrier D) and how a DS0 level line 130, a DS1 level line 132, a DS3 level line 134 and a fiber optic (light) line 136 are used to interconnect a customer X 140 to a customer Y 142. The equipment at the customer premise or site 140 and 142 could be, for example, a telephone, a facsimile modem or a data modem.
A multiplexer/demultiplexer or channel bank 144 is used to multiplex 24 DS0 level signals on the line 130 into one DS1 level signal on the line 132. In this model 120, a M1/3 muldem 146 is used to multiplex 28 DS1 level signals on the line 132 into one DS3 level signal on the line 134. The DS3 level signals on the line 134 are further combined by Carrier A using a lightwave transport multiplexer 122 into a fiber optic signal on the line 136. In this model 120, three Central Offices 152, 154 and 156 are used, with the middle Central Office 154 having three carriers cross-connected at the DS3 level by use of a cross-connect 158.
A long distance call from customer X 140 to customer Y 142 involves many levels of multiplexing and many transport carrier handoffs. Carrier A is the local operating company of customer X 140, and owns Central Offices 152 and 154. Carrier B and Carrier C are long distance carriers, and Carrier D is the local operating company that owns Central Office 156 and services customer Y 142.
A call from customer X 140 to customer Y 142 involves three central offices and three transport carriers. As the call traverses the network 120, it may be processed by several network elements, such as channel banks 144, M1/3 muldems 146, 128, and lightwave transport multiplexers 122, 126 with each element having its own surveillance techniques. Maintenance and billing problems are not uncommon with this interaction.
Most network elements incorporate some form of monitoring, test, and control of the data that they process. However, none of these options supports the continuous monitor or test access of DS3 and all embedded channels.
Although the cost of bandwidth has plummeted to the extent that it no longer worries facility planners as it did in previous decades, the move to DS3 is not without its costs. Chief among them are the lack of convenient and economical test access to lower rate channels embedded in the DS3 bit stream and the lack of surveillance systems designed to take advantage of the performance data embedded in the DS3 formatted signal.
DS3 (and to a lesser extent DS1) signals carry large amounts of data per unit time and represent a considerable financial investment on the part of the end user, for whom bandwidth is not as inexpensive as it has become for the operating company facility planner. The operating company using DS3 runs the risk of a substantial outage in the case of a crippling impairment or total failure of such high-speed digital facilities. Those who manage the DS3 facilities of both end users and service providers are thus quite interested in the performance of the digital links in their networks. They are not satisfied to let the performance information embedded in the bit streams they deal with simply pass on by without extracting data which can be quite useful in managing the network and in minimizing the costly impact of service outages.
It is possible to acquire a DS3 signal at the monitor jack of a DSX-3 cross-connect panel and demultiplex from the DS3 whatever subsidiary signals are desired. Such signals may then be patched into portable test equipment or routed to test systems for analysis. There are many test sets available which will analyze signals extracted at any rate from DS0 to DS3. This technique, however, requires manual access to implement the patching and allows the use of the test and/or surveillance equipment on only one DS3 at a time. Portable test arrangements of this type do not generally allow the insertion of test signals or data into outgoing channels of a DS3 bit stream without interrupting the other services carried by the same DS3.
A digital cross-connect system (DCS) might be considered for use as a test access vehicle in the DS3 network. The versatile and sophisticated switching capabilities of the DCS make it a costly access. There are, in addition, impairments associated with the use of DCS which make it inadvisable to scatter such systems throughout the network at all points requiring surveillance or test access. Among the impairments introduced by a DCS are delay, a certain amount of which is necessary to synchronize incoming and outgoing frame structures, and robbed-bit writeover distortion, the latter difficulty occurring only when switching down to the DS0 rate is provided.
To improve service while cutting costs, RBOCs have turned from portable test equipment in the hands of field craftspeople, to permanently installed test systems connected to a central network management center or operations support system (OSS); and from repair actions in response to a trouble report from customers, to proactive network performance monitoring and preventative maintenance. Existing equipment available to telephone companies provides only a small portion of the functionality needed by telephone companies and is quite expensive.
In a telephone network, a synchronization monitoring function compares the clock frequency of a DS1 signal under test to a reference DS1 signal. The reference signal can be any (embedded or direct interface) DS1 in the test system or an external DS1 based reference received through a test system port.
Synchronization measurements are important because frequency offsets in the network can result in "slips", which are additions or deletions of a DS1 subframe. If all clocks and signal delays were perfectly stable, telephone network timing would not be a problem. Only an initial calibration on the clocks would then be necessary so that they all would operate at the same rate. The telephone network would operate at the same rate indefinitely without timing faults. However, physical devices are not perfect and thus synchronization techniques are utilized. But even with presently known synchronization methods, frequency offsets may still cause problems. Thus it would be a significant advantage for a monitor function to accurately and quickly predict the occurrence of slips in the network.
It would also be desirable to compare the two directions of a selected DS1 signal to determine if the bi-directional paths are synchronized to each other or the external reference. Furthermore, a need exists to establish a threshold level for the allowed number of slips in a specified time period, and when the threshold is exceeded, to send an autonomous alarm message to the network management center where the appropriate action can be taken.
Performance monitoring is performed on bi-directional or through DS3 circuits or channels and all of their constituent sub-circuits commonly known as DS2s and DS1s. These DS3, DS2 and DS1 circuits are organized in the network in a hierarchical structure, with seven DS2s per DS3 and four DS1s per DS2. Pursuant to Bellcore TR-TSY-833 and ANSI T1M1.3 91 specifications, each circuit is monitored for several network disturbance conditions. A disturbance can be a failure, which leads to a loss of service, or a defect, which leads to a degraded state. These disturbances include Loss of Signal (LOS), Loss of Frame (LOF), Alarm Indication Signal (AIS), Yellow Alarm, and so forth, and are issued as autonomous events to the controlling operations support system (OSS), typically via a 9600 baud link using Bellcore TL1 protocol.
In the event of a DS3 level disturbance, the probability of a subordinate or embedded circuit failure is very high. In this scenario, a significant amount of "redundant" information from embedded DS2 and DS1 circuits has the potential of being generated in the form of a flood of autonomous events towards the OSS. In essence, this constituent information is of little value, as only the highest level defect is of concern. In order to provide the OSS with the most pertinent information in identifying the fault, a hierarchical filtering mechanism that reduces the amount of redundant information issued in the event of a higher (DS3 or DS2) level fault would be of great utility.
Consequently, there is a need for DS3 surveillance and testing using a system which is essentially transparent to in-service DS3 lines and paths, which provides non-intrusive surveillance and performance monitoring, which can provide intrusive test access when required and which is economical enough to install at all points requiring DS3 surveillance or test access. There is also a need for network managers to have access to comprehensive performance data embedded in the DS3 format in order to make informed decisions about the operations of their DS3 networks.